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Chapter 10: Problem 53

What is the difference between film and dropwise condensation? Which is a more effective mechanism of heat transfer?

Short Answer

Expert verified
Short Answer: Dropwise condensation is a more effective mechanism of heat transfer than film condensation because it does not have a continuous film insulating the surface, resulting in higher heat transfer rates and better heat transfer efficiency.

Step by step solution

01

Understanding Film Condensation

Film condensation is when a vapor comes into contact with a cold surface and condenses into a continuous film. This layer of condensed liquid acts as an insulating barrier and hinders the heat transfer process. In other words, as the film thickness increases, the thermal resistance of the film increases as well, leading to a decrease in the rate of heat transfer.
02

Understanding Dropwise Condensation

Dropwise condensation, on the other hand, is characterized by the formation of discrete droplets on the cold surface instead of a continuous film. These droplets eventually grow in size, coalesce, and are removed from the surface by gravity or other forces. Since there is no continuous film of liquid acting as an insulator, heat transfer rates are generally higher in dropwise condensation as compared to film condensation.
03

Comparing Heat Transfer Rates

As discussed, dropwise condensation allows for better heat transfer rates due to the absence of an insulating film. This difference can be quantified using the Nusselt number (Nu), which is a dimensionless number used to describe the efficiency of heat transfer in a convective process. In general, dropwise condensation has a higher Nusselt number than film condensation, meaning it has a better heat transfer efficiency.
04

Conclusion

In conclusion, the main difference between film and dropwise condensation lies in the formation of a continuous film in the former, which reduces heat transfer efficiency. Dropwise condensation does not have this continuous film, resulting in higher heat transfer rates. Therefore, dropwise condensation is a more effective mechanism of heat transfer compared to film condensation.

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Most popular questions from this chapter

Saturated water vapor at \(40^{\circ} \mathrm{C}\) is to be condensed as it flows through a tube at a rate of \(0.2 \mathrm{~kg} / \mathrm{s}\). The condensate leaves the tube as a saturated liquid at \(40^{\circ} \mathrm{C}\). The rate of heat transfer from the tube is (a) \(34 \mathrm{~kJ} / \mathrm{s}\) (b) \(268 \mathrm{~kJ} / \mathrm{s}\) (c) \(453 \mathrm{~kJ} / \mathrm{s}\) (d) \(481 \mathrm{~kJ} / \mathrm{s}\) (e) \(515 \mathrm{~kJ} / \mathrm{s}\)

In condensate flow, how is the wetted perimeter defined? How does wetted perimeter differ from ordinary perimeter?

When boiling a saturated liquid, one must be careful while increasing the heat flux to avoid burnout. Burnout occurs when the boiling transitions from boiling. (a) convection to nucleate (b) convection to film (c) film to nucleate (d) nucleate to film (e) none of them

An air conditioner condenser in an automobile consists of \(2 \mathrm{~m}^{2}\) of tubular heat exchange area whose surface temperature is \(30^{\circ} \mathrm{C}\). Saturated refrigerant-134a vapor at \(50^{\circ} \mathrm{C}\) \(\left(h_{f g}=152 \mathrm{~kJ} / \mathrm{kg}\right)\) condenses on these tubes. What heat transfer coefficent must exist between the tube surface and condensing vapor to produce \(1.5 \mathrm{~kg} / \mathrm{min}\) of condensate? (a) \(95 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (b) \(640 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (c) \(727 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (d) \(799 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (e) \(960 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\)

At a distance \(x\) down a vertical, isothermal flat plate on which a saturated vapor is condensing in a continuous film, the thickness of the liquid condensate layer is \(\delta\). The heat transfer coefficient at this location on the plate is given by (a) \(k_{l} / \delta\) (b) \(\delta h_{f}\) (c) \(\delta h_{f g}\) (d) \(\delta h_{g}\) (e) none of them
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